The present invention relates to a method for operating a wind energy plant with a doubly-fed asynchronous machine and to a wind energy plant with a doubly-fed asynchronous machine.
From WO 2005/027301 A1 a method is known for operating a frequency converter for a generator. The method relates to a wind energy plant with a frequency converter which comprises a converter connected to the generator and a converter connected to the grid. Upon considerable decrease of the grid voltage, the method provides that the voltage in an intermediate link between the converters is reduced and that an output current of the grid-sided converter is increased. In addition, the operating frequency for controlling the grid-sided converter can be reduced in order to increase the output current of the grid-sided converter.
From WO 2004/067958 A1 a generator for a wind energy plant is known comprising a low-voltage control for going through grid faults. The control is provided in order to reliably feed power into the grid, wherein in particular the grid codes of the energy supply companies shall be taken into account. The requirements are denoted “low voltage ride through” (LVRT) and demand that a wind energy plant shall continue to feed synchronously into the electric grid if a voltage drop occurs in the grid. In order to comply with these requirements, it is proposed to vary the pitch angles of one or more rotor blades when a voltage drop occurs.
In the past, ever new grid codes for wind energy plants were defined by the energy supply companies, putting specific emphasis on the FRT-characteristics (“fault ride through”) of the wind energy plants. FRT-characteristics of a wind energy plant denotes the wind energy plant's ability to pass through a grid fault without shutdown or disconnection. Numerous different FRT-variants exist, again and again making adaptation of the control of the wind energy plant a necessity. This over and over again requires a redesign of the control of the wind energy plant—in particular of the control of the converters—and a development of new control methods. As a result of the development time and the subsequent testing of the newly adapted control, delays and costs are generated, conflicting with a flexible application of the wind energy plant.
From I. Erlich et al., “Integration of wind power into the German high voltage transmission grid” published in Power Engineering Society General Meeting, 2007, IEEE, Jun. 24, 2007-Jun. 26, 2007, pages 1 to 8, it is known for a doubly-fed asynchronous machine for a wind energy plant to control the rotor-sided converter and the grid-sided converter each with individual electrical parameters. Further, it is known from this document that a disconnection of the asynchronous machine from the grid is allowed only if the grid voltage falls below a predetermined voltage-time-characteristic curve.
The invention is based on the problem to provide a method for operating a wind energy plant with a doubly-fed asynchronous machine, and such a wind energy plant, which can easily be adapted to different FRT-requirements.
The problem is solved by the method with the features of claim 1. The problem is also solved by a wind energy plant with the features of claim 18. Advantageous aspects of the invention are subject to the dependent claims.